The present invention relates to metal-air power supplies. This invention more particularly pertains to ventilation systems for controlling ambient airflow to the oxygen electrodes of metal-air batteries.
Metal-air cells have been recognized as a desirable means for powering portable electronic equipment, such as personal computers, camcorders and telephones, because such battery cells have a relatively high power output with relatively low weight as compared to other types of electrochemical battery cells. Metal-air batteries include an air permeable cathode, commonly referred to as an oxygen electrode, and a metallic anode separated by an aqueous electrolyte. Electrical energy is created with a metal-air battery by an electrochemical reaction.
Metal-air battery cells utilize oxygen from the ambient air as a reactant in the electrochemical process. During discharge of a metal-air battery, such as a zinc-air battery, oxygen from the ambient air is converted at the oxygen electrode to hydroxide, zinc is oxidized at the anode by the hydroxide, and water and electrons are released to provide electrical energy. Metal-air cells utilize oxygen from the ambient air as a reactant, rather than utilizing a heavier material, such as a metal or metallic composition. To operate a metal-air battery, it is therefore necessary to provide a supply of oxygen to the oxygen electrode of the battery.
It is desirable to preserve the efficiency, power and lifetime of a metal-air cell by effectively limiting the transpiration of water vapor between the electrolyte and the atmosphere. Multiple metal-air batteries can be stacked in a common housing to form a battery pack and to isolate the oxygen electrodes. An air mover is used to provide an airflow of ambient air in to the housing of the battery pack to support higher power output. When the air mover is turned on, the air mover circulates ambient air across the oxygen electrodes and forces air through inlet and outlet passageways to refresh the circulating oxygen-depleted air with ambient air, so that oxygen is supplied to the oxygen electrodes. The power output of the battery pack is increased as a result of the flow of ambient air across the metal-air batteries. When the air mover is turned off, airflow across the metal-air batteries is reduced. The reduced airflow amounts to reduced power output.
However, a nominal amount of airflow is still required to maintain an open cell voltage in the battery cells even though power output is no longer desired. During periods of non-use, such as when the battery pack is being stored, the battery pack tends to maintain an equilibrium relative humidity. Thus, if the ambient humidity is greater than the equilibrium humidity within the battery housing, the battery pack will absorb water from the air through the oxygen electrode and fail due to a condition called flooding. On the other hand, if the ambient humidity is less than the equilibrium humidity within the battery housing, the metal-air batteries will release water vapor from the electrolyte through the oxygen electrode and fail due to drying out. Therefore, when the battery pack is not in use, the cells may fail when the level of ambient air humidity differs from the humidity level within the battery housing.
What is needed is a ventilation system for metal-air batteries that keeps water loss or gain to a minimum while also allowing sufficient ambient airflow during discharge so that enough oxygen is present to fuel the electrochemical reaction. For example, U.S. Pat. No. 5,691,074 to Pedicini, entitled xe2x80x9cDIFFUSION CONTROLLED AIR VENT FOR A METAL-AIR BATTERYxe2x80x9d, the entire disclosure of which is incorporated herein by reference, discloses a ventilation system for metal-air batteries. In Pedicini, except for the inlet and outlet passageways, the oxygen electrodes of one or more metal-air battery cells are isolated from the ambient air while the battery cells are not operating. The isolation passageways are sized to (i) pass sufficient ambient airflow while the air mover is operating to enable the metal-air battery cells to provide an output current for powering a load, but (ii) restrict ambient airflow to a low level of diffusion of air while the isolation passageways are unsealed and no ambient air is forced therethrough.
When the air mover is off and the humidity level within the cell is relatively constant, only a very limited amount of air diffuses through the passageways. The water vapor within the cell protects the oxygen electrodes from exposure to oxygen. The oxygen electrodes are sufficiently isolated from the ambient air by the water vapor such that the cells have a long xe2x80x9cshelf lifexe2x80x9d without sealing the passageways. These isolation passageways may be referred to as xe2x80x9cdiffusion tubesxe2x80x9d, xe2x80x9cisolating passagewaysxe2x80x9d, or xe2x80x9cdiffusion limiting passagewaysxe2x80x9d due to their isolation capabilities. Other exemplary isolation passageways and systems are disclosed in U.S. Pat. No. 5,919,582, the entire disclosure of which is incorporated herein by reference.
In accordance with the above-referenced example from Pedicini, the isolation passageways function to limit the amount of oxygen that can reach the oxygen electrodes, which minimizes the self discharge and leakage or drain current of the metal-air battery cells. Self discharge can be characterized as a chemical reaction within a metal-air battery cell that does not provide a usable electric current, but diminishes the capacity of the metal-air battery cell for providing a usable electric current. Self discharge occurs, for example, when a metal-air cell dries out and the zinc anode is oxidized by the oxygen that seeps into the cell during periods of non-use. Leakage current, which is synonymous with drain current, can be characterized as the electric current that can be provided to a closed circuit by a metal-air cell while the cell is connected to the circuit and air is not provided to the cell by an air mover. The isolation passageways as described above may limit the drain current to an amount smaller than the output current by a factor of at least about 50.
The isolation passageways of the Pedicini patent also minimize the detrimental impact of humidity on the metal-air cells, especially while the air mover is not forcing airflow through the isolation passageways. The isolation passageways limit the transfer of moisture into or out of the metal-air cells while the air mover is off, so that the negative impacts of the ambient humidity level are minimized.
The efficiency of the isolation passageways in terms of the transfer of air and water into and out of a metal-air cell can be described in terms of an xe2x80x9cisolation ratio.xe2x80x9d The xe2x80x9cisolation ratioxe2x80x9d is the ratio of the rate of water loss or gain by a cell while its oxygen electrodes are fully exposed to the ambient air, as compared to the rate of the water loss or gain of the cell while its oxygen electrodes are isolated from the ambient air, except through one or more limited openings. For example, given identical metal-air cells having electrolyte solutions of approximately thirty-five percent (35%) KOH in water, an internal relative humidity of approximately fifty percent (50%), the ambient air having a relative humidity of approximately ten percent (10%), and no fan-forced circulation, the water loss from a cell having an oxygen electrode fully exposed to the ambient air should be more than 100 times greater than the water loss from a cell having an oxygen electrode that is isolated from the ambient air, except through one or more isolation passageways of the type described above. In this example, an isolation ratio of more than 100 to 1 should be obtained.
Metal-air cells have found limited commercial use in devices, such as hearing aids, which require a low level of power. In these cells, the air openings which admit air to the oxygen electrode are so small that the cells can operate for some time without flooding or drying out as a result of the typical difference between the outside relative humidity and the water vapor pressure within the cell. However, the power output of such cells is too low to operate devices such as camcorders, cellular phones, or laptop computers. Enlarging the air openings of a typical xe2x80x9cbutton cellxe2x80x9d would lead to premature failure as a result of flooding or drying out.
Systems designed to provide the dual functions of providing air to a metal-air cell for power output and isolating the cells during non-use are referred to as air managers. An important component of a successful air manager is an air mover, such as a fan or an air pump. In the past, air movers used in metal-air batteries have been bulky and expensive relative to the volume and cost of the metal-air cells. While a key advantage of metal-air cells is their high energy density resulting from the low weight of the oxygen electrode, this advantage is compromised by the space and weight required by an effective air mover. Space that could otherwise be used for battery chemistry to prolong the life of the battery must be used to accommodate an air mover. This loss of space can be critical to attempts to provide a practical metal-air cell in small enclosures such as the xe2x80x9cAAxe2x80x9d cylindrical size now used as a standard in many electronic devices. Also, the air mover uses up energy stored in the cells.
One factor increasing the required output characteristics of an air mover for a metal-air cell is the need to overcome the flow resistance of isolating passages of the type described above while maintaining the necessary isolation ratio. To allow smaller power air movers, there is a need for an air manager that permits greater ambient airflow to support higher power output while the metal-air battery cells are in use without making the air mover larger, more expensive to acquire or operate, or require more energy to operate. This new air manager should also restrict the ambient airflow to the extent necessary to protect the cells against excess humidity exchange when the metal-air battery cells are no longer is use.
The present invention alleviates or solves the above-described problems in the prior art by providing an improved ventilation system for metal-air battery cells. The present method and apparatus seeks to provide an efficient method of isolating the oxygen electrodes of metal-air batteries from ambient air when the metal-air battery cells are not in use, while satisfying the need for maximizing ambient airflow to the oxygen electrodes to support higher power output when required, with a low cost, efficient, small air mover.
In accordance with the present invention, this object is accomplished by providing a ventilation system having one or more ventilation passageways. At least a portion of each ventilation passageway is a collapsible isolation passageway for controlling the amount of ambient airflow into a battery housing or a metal-air battery cell. When power is desired, an air mover is turned on to generate ambient airflow into and across the oxygen electrodes. In response to the air mover being turned on, the geometry of the isolation passageways is altered to permit a maximum ambient airflow. When the air mover is turned off, the geometry of the isolation passageways is altered to restrict ambient airflow to substantially isolate the metal-air battery cells when not in use.
Each isolation passageway provides an isolation function while at least partially defining an open communication path between the ambient air and the oxygen electrodes. In some embodiments, however, the isolation passageway embodies the entire ventilation passageway. The isolation passageways regulate the transfer of air and water into and out of the metal-air cell. The transfer of air and water to a fully exposed cell is at least about 50 times greater or more than when the cell is isolated from the ambient air.
In an exemplary embodiment of the present invention, the isolation passageways are biased to normally remain in at least a partially collapsed position to restrict ambient airflow through the battery housing. However, the isolation passageways are expandable in response to the air mover being turned on and ambient airflow passing through the battery housing. When the isolation passageways are in the collapsed position, the isolation passageways have a length in the direction of airflow through the respective isolation passageways that is greater than a width perpendicular to the direction of airflow through the respective isolation passageways. When the air mover is turned on and the isolation passageways are altered, the cross-sectional area or width perpendicular to the direction of the airflow therethrough is enlarged to permit greater ambient airflow into the battery housing and, therefore, greater power output.
Ventilation systems for metal-air cells having isolation passageways formed in accordance with the present invention have a number of advantages. An important advantage of the novel ventilation system is the ability to vary the size of the isolation passageway in response to the operation of the air mover.
Accordingly, an object of this invention is to provide an improved ventilation system for metal-air batteries that overcomes the aforementioned inadequacies of prior art ventilation systems.
Another object of the present invention is to provide a ventilation system for metal-air batteries that permits sufficient airflow during discharge of the metal-air batteries to optimize power output.
Still another object of the present invention is to provide a ventilation system for metal-air batteries that restricts ambient airflow in order to minimize exposure of the metal-air batteries to the atmosphere when the metal-air batteries are not in use.
Yet another object of the present invention is to provide a structurally simple and economical ventilation system for metal-air batteries.
Still a further object of the present invention is to provide a ventilation system for metal-air batteries wherein the operation of the metal-air batteries is transparent to the user. That is, in order to operate the ventilation system of the present invention, no action on part of the user is required.
Yet a further object of the present invention is to achieve and maintain a high isolation ratio.
Another object of the present invention is to minimize the power required to move sufficient air through the isolation passageways.
The foregoing has broadly outlined some of the more significant objects and features of the present invention. These should be construed to be merely illustrative of some of the more prominent features and applications of the intended invention. Many other beneficial results can be obtained by applying the disclosed invention in a different manner or by modifying the disclosed embodiments. Accordingly, other objects and a more comprehensive understanding of the invention may be obtained by referring to the detailed description of the preferred embodiment taken in conjunction with the accompanying drawings, in addition to the scope of the invention defined by the claims. For a more succinct understanding of the nature and objects of the present invention, reference should be directed to the following detailed description taken in connection with the accompanying drawings.